Anatase

Anatase
General
Category Oxide minerals
Formula
(repeating unit)		 TiO2
Strunz classification 4.DD.05
Crystal system Tetragonal
Crystal class Ditetragonal dipyramidal (4/mmm)
H-M symbol: (4/m 2/m 2/m)
Space group I41/amd
Unit cell a = 3.7845, c = 9.5143 [Å]; Z = 4
Identification
Formula mass 79.88 g/mol
Color Black, reddish to yellowish brown, dark blue, gray
Crystal habit Pyramidal (crystals are shaped like pyramids), tabular (form dimensions are thin in one direction).
Twinning Rare on {112}
Cleavage Perfect on [001] and [011]
Fracture Subconchoidal
Tenacity Brittle
Mohs scale hardness 5.5–6
Luster Adamantine to splendent, metallic
Streak pale yellowish white
Diaphaneity Transparent to nearly opaque
Specific gravity 3.79–3.97
Optical properties Uniaxial (-), anomalously biaxial in deeply colored crystals
Refractive index nω = 2.561, nε = 2.488
Birefringence δ = 0.073
Pleochroism Weak
References [1][2][3]

Anatase is a mineral form of titanium dioxide (TiO2). The mineral is almost always encountered as a black solid, although the pure material is colorless or white. Two other mineral forms of TiO2 are known, brookite and rutile.

Anatase is always found as small, isolated and sharply developed crystals, and like rutile, a more commonly occurring modification of titanium dioxide, it crystallizes in the tetragonal system; but, although the degree of symmetry is the same for both, there is no relation between the interfacial angles of the two minerals, except in the prism-zone of 45° and 90°. The common pyramid of anatase, parallel to the faces of which there are perfect cleavages, has an angle over the polar edge of 82°9', the corresponding angle of rutile being 56°52½'. It was on account of this steeper pyramid of anatase that the mineral was named, by René Just Haüy in 1801, from the Greek anatasis, "extension", the vertical axis of the crystals being longer than in rutile. There are also important differences between the physical characters of anatase and rutile: the former is less hard (5.5–6 vs. 6–6.5 Mohs) and dense (specific gravity about 3.9 vs. 4.2). Also, anatase is optically negative whereas rutile is positive, and its luster is even more strongly adamantine or metallic-adamantine than that of rutile.[4]

Crystal habit

A ball-and-stick chemical model of an anatase crystal
Extended portion of the anatase lattice.

Two growth habits of anatase crystals may be distinguished. The more common occurs as simple acute double pyramids with an indigo-blue to black color and steely luster. Crystals of this kind are abundant at Le Bourg-d'Oisans in Dauphiné, where they are associated with rock-crystal, feldspar, and axinite in crevices in granite and mica-schist. Similar crystals, but of microscopic size, are widely distributed in sedimentary rocks, such as sandstones, clays, and slates, from which they may be separated by washing away the lighter constituents of the powdered rock.[4]

Crystals of the second type have numerous pyramidal faces developed, and they are usually flatter or sometimes prismatic in habit; the color is honey-yellow to brown. Such crystals closely resemble xenotime in appearance and, indeed, were for a long time supposed to belong to this species, the special name wiserine being applied to them. They occur attached to the walls of crevices in the gneisses of the Alps, the Binnenthal near Brig in canton Valais, Switzerland, being a well-known locality. Naturally occurring pseudomorphs of rutile after anatase are also known.[4]

While anatase is not an equilibrium phase of TiO2, it is stable near room temperature. At temperatures between 550 and about 1000 °C, anatase converts to rutile. The temperature of this transformation strongly depends on the impurities or dopants as well as on the morphology of the sample.[5]

Synthetic anatase

Due to its potential application as a semiconductor, anatase is often prepared synthetically. Crystalline anatase can be prepared in laboratories by chemical methods such as sol-gel method. Examples include controlled hydrolysis of titanium tetrachloride (TiCl4) or titanium ethoxide. Often dopants are included in such synthesis processes to control the morphology, electronic structure, and surface chemistry of anatase.[6]

Alternate and obsolete names

Another name commonly in use for this mineral is octahedrite, a name which, indeed, is earlier than anatase, and given because of the common (acute) octahedral habit of the crystals. Other names, now obsolete, are oisanite and dauphinite, from the well-known French locality.[4]

See also

References

  1. ↑ Anatase, Handbook of Mineralogy
  2. ↑ Anatase, Mindat.org
  3. ↑ "Anatase". Webminerals. Retrieved 2009-06-06.
  4. 1 2 3 4  One or more of the preceding sentences incorporates text from a publication now in the public domain: Spencer, Leonard James (1911). "Anatase". In Chisholm, Hugh. Encyclopædia Britannica. 1 (11th ed.). Cambridge University Press. pp. 919–920.
  5. ↑ Hanaor, Dorian A. H.; Sorrell, Charles C. (2011). "Review of the anatase to rutile phase transformation" (PDF). Journal of Materials Science. 46 (4): 855–874. doi:10.1007/s10853-010-5113-0.
  6. ↑ Jeantelot, Gabriel; Ould-Chikh, Samy; Sofack-Kreutzer, Julien; Abou-Hamad, Edy; Anjum, Dalaver H.; Lopatin, Sergei; Harb, Moussab; Cavallo, Luigi; Basset, Jean-Marie (2018). "Morphology control of anatase TiO2 for well-defined surface chemistry". Physical Chemistry Chemical Physics. 20 (21): 14362–14373. doi:10.1039/C8CP01983E. PMID 29767182. Retrieved 10 July 2018.
This article is issued from Wikipedia. The text is licensed under Creative Commons - Attribution - Sharealike. Additional terms may apply for the media files.